For industrial PSA tape applications it is very common to use polyacrylate PSAs. Polyacrylates possess a variety of advantages over other elastomers. They are highly stable toward UV light, oxygen, and ozone. Synthetic and natural rubber adhesives normally contain double bonds, which make these adhesives unstable to the aforementioned environmental effects. Another advantage of polyacrylates is their transparency and their serviceability within a relatively wide temperature range.
Polyacrylate PSAs are generally prepared in solution by free radical polymerization. The polyacrylates are generally applied to the corresponding backing material from solution using a coating bar, and then dried. In order to increase the cohesion, the polymer is crosslinked. Curing takes place thermally or by UV crosslinking or by EB curing (EB: electron beams). The process described is relatively costly and ecologically objectionable, since as a general rule the solvent is not recycled and the high consumption of organic solvents represents a high environmental burden.
Moreover, it is very difficult to produce PSA tapes with a high adhesive application rate without bubbles.
One remedy to these disadvantages is the hotmelt process. In this process, the PSA is applied to the backing material from the melt.
However, this new technology has its limitations. Prior to coating, the solvent is removed from the PSA in a drying extruder. The drying process is associated with a relatively high temperature and shearing effect, so that high molecular mass polyacrylate PSAs in particular are severely damaged. The acrylic PSA gels, or the lower molecular mass fraction is greatly enriched as a result of molecular weight breakdown. Both effects are undesirable, since they are disadvantageous for the application. Either the adhesive can no longer be applied, or there are changes in its technical adhesive properties, since, for example, when a shearing force acts on the adhesive the lower molecular mass fractions act as lubricants and so lead to premature failure of the adhesive.
One solution to mitigating these disadvantages is offered by polyacrylate adhesives with a low average molecular weight and narrow molecular weight distribution. In this case the fraction of low molecular mass and high molecular mass molecules in the polymer is greatly reduced by the polymerization process. The reduction in the high molecular mass fractions reduces the flow viscosity, and the adhesive shows less of a tendency to gel. As a result of the reduction in the low molecular mass fraction, the number of oligomers which reduce the shear strength of the PSA is lessened.
A variety of polymerization methods are suitable for preparing low molecular mass PSAs. The state of the art is to use regulators, such as alcohols or thiols, for example (Makromolekule, Hans-Georg Elias, 5th Edition, 1990, Hüthig & Wepf Verlag, Basel). These regulators reduce the molecular weight but broaden the molecular weight distribution.
Another controlled polymerization method used is atom transfer radical polymerization (ATRP), in which initiators used preferably include monofunctional or difunctional secondary or tertiary halides and, for abstracting the halide(s), complexes of Cu, Ni, Fe, Pd, Pt, Ru, Os, Rh, Co, Ir, Cu, Ag or Au [EP 0 824 111 A1; EP 0 826 698 A1; EP 0 824 110 A1; EP 0 841 346 A1; EP 0 850 957 A1]. The various possibilities of ATRP are further described in U.S. Pat. Nos. 5,945,491, 5,854,364 and 5,789,487. Generally, metal catalysts are used, which have the side effects of adversely influencing the aging of the PSAs (gelling, transesterification). Moreover, the majority of metal catalysts are toxic, discolor the adhesive, and can be removed from the polymer only by complicated precipitations. A further variant is the RAFT process (reversible addition-fragmentation chain transfer). The process is described at length in WO 98/01478 A1 and WO 99/31144 A1, but in the manner set out therein is unsuited to the preparation of PSAs, since the conversions achieved are very low and the average molecular weight of the polymers prepared is too low for acrylic PSAs. Accordingly, the polymers described cannot be used as acrylic PSAs.
Improvements to the preparation process through the introduction of thioesters of trithiocarbonates are a subject of research. Nevertheless, acrylic PSAs comprising thioesters or trithiocarbonates generally have disadvantages for numerous fields of use. In the case of crosslinking with electron beams (necessary for acrylic hotmelts, applied at a high rate), irradiation is accompanied by the formation of sulfur fragments, which give rise to a very unpleasant odor. This must absolutely be avoided for PSA tapes.
U.S. Pat. No. 4,581,429 discloses a controlled radical polymerization process. As its initiator the process employs a compound of the formula R′R″N—O—X, in which X denotes a free radical species which is able to polymerize unsaturated monomers. In general, however, the reactions have low conversion rates. A particular problem is the polymerization of acrylates, which takes place only with very low yields and molecular weights.
WO 98/13392 A1 describes open-chain alkoxyamine compounds which have a symmetrical substitution pattern. EP 735 052 A1 discloses a process for preparing thermoplastic polymers having narrow polydispersities.
WO 96/24620 A1 describes a polymerization process in which very specific radical compounds, such as phosphorus-containing nitroxides, for example, are described.
WO 98/30601 A1 discloses specific nitroxyls, based on imidazolidine.
WO 98/4408 A1 discloses specific nitroxyls, based on morpholines, piperazinones and piperazinediones.
DE 199 49 352 A1 discloses heterocyclic alkoxyamines as regulators in controlled radical polymerizations.
Corresponding further developments of the alkoxyamines or of the corresponding free nitroxides improved the efficiency for the preparation of polyacrylates [Hawker, C. J., paper, National Meeting of the American Chemical Society in San Francisco, Spring 1997; Husemann, M., IUPAC World-Polymer Meeting 1998, Gold Coast, Australia, paper on “Novel Approaches to Polymeric Brushes using ‘Living’ Free Radical Polymerizations” (July 1998)]
In the abovementioned patents and papers attempts were made to improve the control of radical polymerization reactions. There nevertheless exists a need for a nitroxide-controlled polymerization process which is highly reactive and can be used to realize high conversions in combination with high molecular weight and low polydispersity.
Experiments relating to such techniques require highly inert conditions; moreover, only purified and distilled monomers can be used. At the present time, this process is difficult to scale up to an economic industrial process.
U.S. Pat. No. 6,166,155 and WO 98/11143, in contrast, describe processes for polymerizing vinyl compounds which provide polymers with polydispersities of less than 2. Here, electron donor compounds and triazolyl radicals, respectively, are used as control reagents for the polymerization. Acrylic PSAs are not described therein. Moreover, some of the compounds described therein contain sulfur or selenium, which again, in the case of electron beams for crosslinking these acrylic PSAs, would lead to fragmentation. Furthermore, acrylic PSA tapes are subject to very strict regulations, and so PSA tapes containing selenium can be ruled out per se.
Additionally, polydispersities of less than 2 are very difficult to achieve for acrylic PSAs, since the polymerizations generally have to be carried out up to a high conversion (>98%). In order to bring this about within a relatively short time, it is necessary to add two or more initiators, which accelerate the reaction and thus impair the effect of the control reagent. A polydispersity range from 2 to 3.5 is therefore regarded as ideal.
It is an object of the invention, therefore, to provide an initiator system for a corresponding polymerization process, and to offer a polymerization process, which does not have the disadvantages of the aforementioned prior art, or at least not to so great an extent.